In the manufacture of paper webs, such as tissue webs, a slurry of cellulosic fibers is deposited onto a forming wire to form a wet embryonic web. The resulting wet embryonic web may be dried by any one of or combinations of known means, where each drying means may potentially affect the properties of the resulting tissue web. For example, the drying means may affect the softness, caliper, tensile strength, and absorbency of the resulting cellulosic tissue web.
An example of one drying means is through-air drying. In a typical through-air drying process, a foraminous air permeable fabric supports the embryonic web to be dried. Hot air flow passes through the web, then through the permeable fabric or vice versa. The air flow principally dries the embryonic web by evaporation. Regions coincident with and deflected into fabric voids are preferentially dried. Regions of the web coincident with solid regions of the fabric, such as woven knuckles, are dried to a lesser extent by the airflow as the air cannot pass through the fabric in these regions.
To improve the efficiency and effectiveness of through-air drying several improvements to through-air drying fabrics have been made. For example, the in certain instances the air permeability of the fabric has been increased by manufacturing the fabric with a high degree of open area. In other instances fabrics have been impregnated with metallic particles to increase their thermal conductivity and reduce their emissivity. In still other instances the fabric itself has been manufactured from materials specially adapted for high temperature airflows. Examples of such through-air drying technology are found, for example, in U.S. Pat. Nos. 4,172,910, 4,251,928, 4,528,239 and 4,921,750.
While the foregoing fabric improvements have resulted in certain beneficial gains, they have not yet successfully addressed problems associated with through-air drying non-uniform tissue webs. For example, a tissue web having a first region with lesser absolute moisture, density or basis weight than a second region, will typically have relatively greater airflow through the first region compared to the second. This relatively greater airflow occurs because the first region of lesser absolute moisture, density, or basis weight presents a proportionately lesser flow resistance to the air passing through such region. As a result the first and second regions dry at different rates and may ultimately result in a web having variable moisture content and/or physical properties.
Drying of the paper web is often rate limiting and is dependent upon the drying time and the drying rate. Decreasing the drying time typically requires increases in the dimensions of the dryer, which is capital intensive, and therefore papermakers often seek to maximize the drying rate to improve drying. The drying rate (R in g/m{circumflex over ( )}2/s) in a typical papermaking process is described by:
                    R        =                              h            φ                    ⁢                      (                                          T                supply                            -                              T                sheet                                      )                                              (                  Equation          ⁢                                          ⁢          1                )            Where h is the heat transfer coefficient (having units of W/m2 K), φ is the latent heat of water evaporated during drying, Tsheet is the temperature of the web and Tsupply is the air temperature of the air supplied the dryer. The heat transfer coefficient is influenced by the mass of air contacting the web during the drying process. The latent heat (φ) of the water evaporated during drying is typically about 2265 joules per gram (j/g) and is constant for a given web temperature. The temperature of the web begins at the wet bulb temperature when the web is wet and rises to the temperature of the heated dryer air.
To improve the efficiency of through-air drying the supply temperature is often increased. The maximum supply temperature however is limited by several factors such as the ignition temperature of the sheet and the melting temperature of the carrying fabric. For example, webs made from wood pulp fibers may begin to degrade when the web temperature exceeds 300° F. and produce off odors and polyester, which is commonly used in the manufacture of carrying fabrics, undergoes hydrolysis at about 350° F. and melts at 480° F.
To overcome these limitations, the prior art has often resorted to alternative through-air dryer designs and the introduction of alternate drying medium. For example, U.S. Pat. No. 6,732,452 teaches the addition of high temperature steam to the drying medium to increase the supply temperature and eliminate the scorching or burning of the drying web. Such methods however, often introduce complexities to the manufacturing process and require additional capital improvements.
Thus, there remains a need in the art for more efficient through-air drying processes, particularly processes that can accommodate non-uniform tissue webs and the use of fabrics having varying degrees of air permeability. Further there is a need for a means of increasing the supply temperature using existing through-air drying apparatuses without damaging the nascent web or negatively affecting important web properties.